Concrete is generally supplied to a delivery site from a concrete batching plant located off the site, usually at the supplier's place of business. There, the required ingredients are weighed out and mixed, then loaded into a concrete mixer truck for delivery to the customer.
The ingredients are crushed stone and/or gravel, sand, cement, water and certain additives.
Immediately upon entering in contact, the cement and the water start a chemical reaction, the ultimate result of which is the hydration of the concrete, its "setting" or "hardening".
Because of this reaction, and to avoid segregation of the ingredients during transport, the concrete is agitated in a revolving mixer drum mounted on the truck, until discharged at the job site. A concrete truck is therefore a very costly piece of equipment.
Also because of the aforementioned reaction, this so called ready-mix concrete is the most perishable cargo carried in any quantity in modern urban traffic. Foodstuffs, ice, flowers etc. all last in good condition for more than 90 minutes, which is the generally accepted time limit for condemning a load of concrete.
As the average strength and quality of concrete keeps going up, so does the stringency of rules by which the owner's engineers try to protect that quality. What in bygone days was tolerated for the weaker mixes cannot be permitted for today's higher strength concretes. The sensitivity of future concretes to the requirements established for quality concrete can be predicted to increase not least in what concerns delivery time, already counted in minutes.
Meanwhile the conditions in which ready-mix concrete is delivered are becoming progressively worse. Traffic density increases, the number of stop signs and traffic lights constantly grows, routes on which truck traffic is allowed are progressively more restricted, limitations are legislated reducing vehicle loads, etc. None of these tendencies work in the concrete suppliers' favour.
On top of the above mentioned problems, the ready-mix concrete business in most areas suffers from a seasonal feast-and-famine pattern. While August to October the industry is hard pressed to find enough delivery capacity to satisfy the market, it has its yards full of idle equipment from January to March, due to the seasonal nature of its clients' business, in turn imposed by the climate.
Another weekly cycle superimposes itself on this annual cycle. Most construction sites so organize their work that placing of concrete is done on Thursdays and Fridays. This is so because thus the hydration process can take place over the weekend, on idle time. When the crews return on Monday morning, stripping of forms can be undertaken without any loss of working time, the setting of the concrete having occurred on Saturday and Sunday. Therefore, even in the busy seaason, ready-mix suppliers find themselves saddled with idle equipment and underutilized batch plants on Mondays, Tuesdays and Wednesdays, while the next two days see heavy overtime costs incurred.
The industry thus finds itself in a situation, where a cheap product (value just over 1.cent./lb), is transported in very expensive and complicated equipment in conditions which render it rather precarious to meet delivery times imposed by the extreme perishability of the product. The large investment in equipment is in full use only two days a week under 3 or 4 months of the year and must thus be amortized over a few annual hours only. Obviously, some place along the road this industry either took a wrong turn, or more likely, failed to make a turn it should have.
U.S. Pat. No. 3,180,686, of 1965, has proposed rendering the concrete product non-perishable by not adding water to it until the time of its use at the terminal site. In this Patent, a trailer truck transports the aggregate in one compartment and cement in another compartment to the delivery site, where the ingredients are discharged onto a conveyor which feeds the dry material to a concrete mixer where water is added. However, in this Patent, the dry aggregate is exposed to the elements and to humidity infiltration or evaporation during transport so that there are no means to ascertain the degree of its humidity when arriving at the construction site and therefore it is impossible to calculate the exact amount of water to be added in the concrete mixer for obtaining the specified hydration and strength of the concrete. Furthermore the patent requires that the cement compartment have its discharge opening situated above that of the aggregate compartment, thereby restricting the flow of the aggregate.